A color electrophotographic device includes a plurality of photosensitive drums 1 as shown in FIG. 2 explaining an image forming apparatus of the present invention. Each photosensitive drum 1 is supported rotatably and driven to rotate in a direction indicated by an arrow by means of a driving device.
Provided sequentially around each photosensitive drum 1 are: a charging device 2 for charging the surface of the photosensitive drum 1 uniformly; exposing means 3 for exposing the surface of the photosensitive drum 1 to form a latent image; a developing device 4 for developing the latent image to a toner image; a transferring device 5 for transferring the toner image onto a transfer material; and a cleaning device 6 for cleaning residual toner on the surface of the photosensitive drum 1.
The four photosensitive drums 1 are aligned in series, and provided with developing bathes respectively containing color toners: Y (yellow), M (Magenta), C (Cyan), and Bk (Black).
The transferring device 5 is provided with a transfer belt 7 which is looped over a driving pulley and a slave pulley and run in a direction indicated by arrows. A recording medium as the transfer material is held electrostatically and thus transported by the transfer belt 7.
Transfer chargers 8 are provided inside the loop of the transfer belt 7 so as to oppose the respective photosensitive drums 1, thereby transferring toner images formed on their respective photosensitive drums 1 onto the recording medium.
The recording medium held by the transfer belt 7 passes by the four photosensitive drums 1 and further a fusing device 9, whereby the toner image is fused onto the same.
In the electrophotographic device represented by the above color electrophotographic device, density characteristics vary with an applied voltage and a temperature rise caused by a series of charging, exposing, and developing actions in the electrophotographic process. For this reason, before input image data is outputted, the density characteristics are corrected as desired in most of the cases.
More specifically, a plurality of toner patterns each having a specific density are formed on the photosensitive drum 1 each time the power supply starts or a predetermined number of sheets are released. Then, the densities of these toner patterns are checked by a sensor, and differences between the checked densities and corresponding optimal densities, that is, the preset reference values, are found. Then, a density conversion table is created based on these differences. Thereafter, the density of the input image data is corrected based on the density conversion table thus created before the image is outputted.
Incidentally, an image forming apparatus, such as an electrophotographic device, outputs several kinds of images including solid (black-painted), line, half-tone, etc. Images vary when influenced by varying factors, namely, a series of charging, exposing, developing, and transferring actions in the electrophotographic process, and therefore, the density of an output image can not be maintained at a specific level without effecting a density correction. In addition, influence from the varying factors in the electrophotographic process varies with the kinds of images, and a density correction has to be controlled differently for each kind of images.
However, because the conventional image forming apparatus controls a quantity of toner to be adhered to the photosensitive drum 1 by detecting the density or a quantity of toner on a solid image, there rises a problem that a density of an entire output image can not be made uniform for each kind of images.
To solve the above problem, for example, Japanese Laid-open Patent Application No. 265571/1996 (Japanese Official Gazette, Tokukaihei No. 8-265571, publishing date: Oct. 11, 1996) discloses an image forming apparatus as follows.
That is, the image forming apparatus disclosed in the above publication forms two kinds of density detecting toner patterns, namely, a high density detecting toner pattern and a half-tone density toner pattern, by changing a developing bias, so that the density of an entire image having half-tones, etc. can be corrected adequately.
The high density detecting toner pattern is detected by toner quantity detecting means composed of a reflection sensor, and the maximum toner quantity on the photosensitive drum is controlled by density correcting means. The half-tone detecting toner pattern is also detected by the toner quantity detecting means, and a toner quantity on the photosensitive drum for a half-tone density portion is adjusted under control of the density correcting means. By effecting both the control of the maximum toner quantity using the high density detecting toner pattern and the control of the toner quantity for the half-tone density portion using the half-tone detecting toner pattern, the density of an entire image is maintained uniformly at a specific level.
More specifically, the density correcting means has the toner quantity detecting means detect the high density from the high density detecting toner pattern, compares the detected high density with the high density reference value, and effects the high density correction if there is a difference. Further, after the high density portion of the image is adjusted, the density correcting means has the toner quantity detecting means detect the half-tone density from the half-tone density detecting toner pattern, compares the detected half-tone density with the half-tone density reference value, and effects the half-tone density correction if there is a difference.
Also, Japanese Laid-open Patent Application No. 289148/1996 (Japanese Official Gazette, Tokukaihei No. 8-289148, publishing date: Nov. 1, 1996) discloses a half-tone density correction. According to this publication, a plurality of half-tone toner patterns of respective colors are formed on a transfer belt by changing a pulse width of a laser beam during the exposing action in the series of charging, exposing, developing, and transferring actions, and these patterns are checked sequentially by a density detecting sensor, whereby an optical density is detected from the checking results.
Then, the checked density values are compared with prestored target density values of their respective colors, and differences between the target density values and current density values are computed to update a half-tone table.
As has been discussed, the density correction using the toner patterns is effected by forming a plurality of toner patterns each time the power supply starts or a predetermined number of sheets are released.
Hence, the conventional image forming apparatus not only effects the high density correction, but also adjusts the half-tone density, and therefore, the density correction takes a considerably long time. Consequently, the conventional image forming apparatus keeps the user waiting longer compared with an image forming apparatus which effects the high density correction alone, thereby posing a problem as not being user-friendly.
In addition, because the half-tone density is adjusted in addition to the high density correction, a larger quantity of toner is consumed in forming the half-tone density detecting toner patterns for the half-tone density adjustment, thereby increasing the cost undesirably.
The following will explain more in detail the half-tone density correction method disclosed in Japanese Laid-open Patent Application No. 289148/1996 supra. Initially, a first density conversion table used as a reference is created at the time of maintenance. Then, when an image is outputted, a toner pattern is formed each time a predetermined number of copies are made, and the density of the toner pattern is checked, based on which a second density conversion table is created. Subsequently, a new density conversion table for use in actual image output is created based on the first and second density conversion tables. In other words, for a middle density area, an interior division of the first and second density conversion tables is used, and for a low/high density area, table values are set linearly with a certain gradient with reference to the neighboring middle density area.
Generally, forming a toner pattern and checking the density thereof involves various kinds of treatments of the photosensitive body, developing device, sensor in the transferring device, etc. However, the density of the toner pattern may include noise for various reasons during these treatments. In such a case, if the density conversion table is created on the assumption that the detected density of the toner pattern is completely correct, an output image may be formed with a density at an incorrect level. Also, the number of toner patterns is generally smaller than that of the levels of the output image density, and for this reason, a density conversion table covering all the levels is created by means of interpolation. However, if a density conversion table is created incorrectly by means of interpolation, an output image may be formed with a density at an incorrect level.
Further, in order to complete the interpolation precisely, a significant length of time is required and there arises a problem that such an image forming apparatus can not be put into practical use.
On the other hand, in case of a multi-color image formed by superimposing images of respective colors, if a correction is made based on a density difference in only one color as is in the conventional method, variance in a color balance of the entire multi-color image can not be detected accurately.
In other words, a color balance of the entire color image varies markedly with ambient temperature and humidity, and life characteristics of members involved in image formation. Thus, in order to output a color image, it is essential to judge variance in a color balance of the entire color image and correct the same.
In addition, an image density varies with the foregoing varying factors whether in the high density area or half-tone density area, and the variance often differs significantly among colors.
Conventionally, in order to solve the above problem, whether a correction action should be effected or not is determined by variance in only one color, that is, when a difference from the reference density becomes greater than a predetermined value. In other words, as shown in FIG. 12, when an image density supposed to be within an area A indicted by diagonal lines is actually in an area B.
However, if a member involved in image formation is replaced, as shown in FIG. 13, the density may change from a density curve before replacing a member (indicated by an alternate long and short dash line) to a density curve after replacing a member (indicated by an alternate long and two short dashes line). In such a case, variance in a color balance can not be judged precisely.
In other words, FIG. 13 shows the density change of only one color, but each color varies differently with ambient temperature and humidity, and the life characteristics of the members. Thus, even if a difference from the reference density is small in each color, when a toner image of each color is superimposed to form a multi-color image, a color balance of the resulting color image often varies.
Thus, whether a correction action should be effected or not is conventionally determined by variance in only one color as shown in FIG. 12, but variance of a color image can not be judged precisely by this conventional method.